The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for reducing electromagnetic interference (EMI). Merely by way of example, the invention has been applied to a switch mode power converter. But it would be recognized that the invention has a much broader range of applicability.
Switch mode power converters include at least flyback power converters and forward power converters. The flyback power converters often use primary-side sensing and regulation and can provide cost-effective solutions for low power applications.
FIG. 1 is a simplified conventional flyback power converter system. For example, the flyback power converter system 100 uses a primary-side sensing and regulation scheme. In another example, the primary circuit of the system 100 includes a primary switch that is controlled through a feedback control signal VFB. In yet another example, the primary circuit is also called the input circuit.
The system 100 also includes a transformer that includes an input winding 110, an output winding 120, a feedback sensing winding 130, and an IC VDD supply winding 140. For example, the feedback sensing winding 130 and the IC VDD supply winding 140 are two windings. In another example, the feedback sensing winding 130 and the IC VDD supply winding 140 are one same winding.
For example, the input winding 110 is on the primary side, and the windings 120, 130, and 140 are on the secondary side. For each of these four windings, a winding dot polarity is used to indicate the phase relationship of the winding voltages. For example, during voltage transitions across the windings, the dot ends of the windings are in phase. In another example, through the feedback sensing winding 130, the feedback control signal VFB is a replica of an output voltage Vo. Additionally, the system 100 may include a clamp circuit, which may be a RCD snubber. In yet another example, the system 100 includes a switch 150.
During the normal operation of the flyback power converter system 100, the voltages across both the input winding 110 and the output winding 120 change accordingly. These changes often generate displacement currents in various parasitic capacitors. These displacement currents usually have a common-mode (CM) component and a differential-mode (DM) component. The DM component often can be filtered by, for example, a front-end filter. But the CM component usually cannot be filtered effectively, and can thus cause significant EMI issues as measured by, for example, a Line Input Stabilization Network (LISN).
FIG. 2 shows simplified conventional parasitic capacitors that exist between an input winding and an output winding, and between a transformer core and an electrical earth. For example, FIG. 2 shows simplified conventional parasitic capacitors that exist in the system 100. In another example, the transformer core is a ferrite core used in the transformer construction to provide a low reluctance path for the magnetic flux coupling the input and output windings of the transformer.
As shown in FIG. 2, there may be common-mode and differential-mode displacement currents. The Y-capacitor Cy often is used in a switch mode power converter to reduce the common-mode EMI. For example, the Y-capacitor Cy provides a low-impedance path for the displacement current that flows between the input circuit and the output circuit of a flyback power converter and returns to its source without flowing through the electrical earth. The current in the Y-capacitor Cy is not detected by the LISN; hence the use of a Y-capacitor can reduce the common-mode EMI.
But the use of a Y-capacitor can create a cost issue for certain low-power AC/DC systems. Without such Y-capacitor, these low-power systems may suffer from serious EMI problems. For example, in the low-power systems, displacement currents are generated by the operation of switch mode power converters. These displacement currents often flow to the electrical earth and cause EMI to other equipment. Hence, it is desirable to keep these currents below limits, such as ones set by various regulatory agencies. In more detail, the switch mode power converters include transformers, which can generate at least two displacement currents that flow to the electrical earth. One displacement current flows from the core of a transformer to the electrical earth. For example, this current is generated by voltage transitions on the transformer windings that couple to the core of the transformer by parasitic capacitors, and hence flows through free space between the core of the transformer and the electrical earth. In contrast, the other displacement current is generated by differential voltages between the primary winding and the secondary winding of the transformer. For example, the differential voltages generate the displacement current in the inter-winding capacitors, and this displacement current returns to its source through parallel paths.
To reduce displacement currents, several conventional methods and apparatuses have been developed. Some of these methods and apparatuses use the conventional techniques of winding cancellation or winding balancing in an energy transfer element, such as a transformer. For example, the techniques of winding cancellation or winding balancing can reduce the displacement current between the coupling of the input winding and the output winding. Specifically, a cancellation winding is coupled to the input ground, and is physically positioned within the transformer and close to the output winding. Therefore, the cancellation winding can generate the displacement current through inter-winding parasitic capacitors. If the displacement current, which flows from the cancellation winding to the output winding, exactly matches the displacement current that flows from the output winding to the input ground through an electrically conductive shield, such as copper, the net displacement current is zero.
Additionally, the conventional techniques of winding cancellation can also be used to reduce the displacement current flowing between the primary winding and the transformer core that often is made of one or more ferrite materials. The net effect of adding cancellation winding is that the influence of the electrostatic field produced by other windings in the transformer relative to the transformer core can be cancelled by the electrostatic field created by the cancellation winding. Consequently the displacement current between transformer windings and transformer core can be reduced theoretically to zero if the electrostatic fields perfectly cancel.
The conventional techniques of winding cancellation or winding balancing may require a large number of winding turns. Hence it is highly desirable to improve techniques for reducing electromagnetic interference.